1. Field of the Invention
The present invention relates to a method and a system for controlling a continuously variable V-belt transmission.
2. Description of the Prior Art
A continuously variable V-belt transmission is known from laid-open Japanese patent application No. Sho 55-65755 which corresponds to Australian Pat. No. 52,356/79, wherein a continuously variable ratio change is accomplished by adjusting V-shaped pulley groove of a drive pulley and a driven pulley to change the radius of the diameter at which a V-belt contacts each of the pulleys. It is a common practice to use a shift control valve as shown in FIG. 1A to distribute oil pressure between a drive pulley cylinder chamber and a driven pulley cylinder chamber. This shift control valve comprises a valve bore 201 formed with ports 201a, 201b, 201c, 201d and 201e, and a corresponding spool 202 to the valve bore 201 having lands 202a, 202b, 202c and 202d. Line pressure P.sub.L is supplied to the center port 201c, and the ports 201b and 201d on either side of the center port are connected to a drive pulley cylinder chamber and a driven pulley cylinder chamber, respectively, and the ports 201a and 201e are both drained off. The spool 202 has its left end, as received in FIG. 1A, linked with for example a shift operating mechanism which adjusts the axial position of the spool. The axial lengths of the lands 202b and 202c of the spool 202 are shorter than axial lengths of the ports 201b and 201d, respectively, and an axial distance l.sub.1 between the outer side edge faces of the lands 202b and 202c is shorter than an axial distance L between outer side walls of the ports 201b and 201c. This structure permits the distribution of the line pressure from the port 201c between the ports 201b and 201d. A part of oil that flows to the ports 201b and 201d through clearances 203a and 204a, respectively, passes through clearances 203 b and 204b to the ports 201a and 201e to be drained off. Therefore, the oil pressure at the port 201b and that at the port 201d are dependent upon the ratio between the clearances 203a and 203b and the ratio between the clearances 204a and 204b, that is, the oil pressure at the port 201b increases as the clearance 203a increases and the clearance 203b decreases and the oil pressure at the port 201d increases as the clearance 204a increases and the clearance 204b decreases. However, as the clearance 203a increases to cause the clearance 203b to decrease, the clearance 204a decreases to cause the clearance 204b to increase, resulting in the oil pressure at the ports 201b and 201d varying as shown in FIG. 1D where the variation in oil pressure is illustrated vs., stroke of the spool 202 along the abscissa. When the spool 202 has moved to the left limit position as shown in FIG. 1B, the oil pressures at the ports 201b and 201d are indicated by a point A shown in FIG. 1D and thus the line pressure appears at the port 201b and the pressure at port 201d is drained off. From this position if the spool 202 is moved gradually to the right, the oil pressure at the port 201b drops and the oil pressure at the port 201d increases. When the spool 202 is at the center position (the position shown in FIG. 1A), the oil pressures are as indicated by B shown in FIG. 1D, resulting in that the oil pressures at the ports 201b and 201d being half of the line pressure. If the spool 202 is moved further to the right and reaches the position shown in FIG. 1C, the oil pressures are as indicated by the point C in FIG. 1D, resulting in the port 201b being drained off and the line pressure P.sub.L appearing at the port 201d. As is apparent from FIG. 1D, in response to the stroke of the spool 202, the oil pressure at the ports 201b and 201d, viz., the oil pressure within the drive pulley cylinder chamber and that within the driven pulley cylinder chamber, vary correlatedly, thus effecting a shift in accordance with the ratio between these oil pressures.
However, such conventional shift control valves, are designed so that the axial distance L between the ports is longer than the axial distance l.sub.1, and always form clearances through which the line pressure is drained off whichever position the spool assumes. Thus an oil pump with a capacity high enough for compensating the leak of the oil pressure must be provided, and furthermore since the oil pressures supplied to the drive pulley cylinder chamber and the driven pulley cylinder chamber drop to half of the line pressure when for example the oil pressures within both of the pulley cylinder chambers are equal to each other (point B), it is difficult to ensure sufficient transmission torque capacity unless the level of the line pressure is kept high.